This invention relates generally to processes for making a bioderived propylene glycol (1,2-propanediol). More particularly, the present invention relates to methods for making a bioderived propylene glycol through the reaction of a biobased feedstock—conventionally, glycerol, five and six carbon sugars and/or sugar alcohols and/or lactic acid—with hydrogen to provide a polyol product mixture including propylene glycol, wherein the polyol product mixture is further processed to yield a commercially acceptable biobased equivalent to the petroleum-based or -derived commodity propylene glycol product used today in so many different applications.
The present invention is thus generally concerned with the development of renewably sourced products which are able to serve as commercially acceptable replacements for materials, and especially commodities such as propylene glycol and ethylene glycol, which are presently largely made downstream of conventional fossil fuel operations. Such biobased, renewably sourced materials can be differentiated from their petroleum-derived counterparts, for example, by their carbon isotope ratios using ASTM International Radioisotope Standard Method D 6866, the disclosure of which is incorporated by reference in its entirety. Method D 6866 is based upon the fact that isotopic ratios of the isotopes of carbon within any given material, such as the 13C/12C carbon isotopic ratio or the 14C/12C carbon isotopic ratio, can be determined using certain established analytical methods, such as isotope ratio mass spectrometry, with a high degree of precision.
ASTM Method D6866, similar to radiocarbon dating, compares how much of a decaying carbon isotope remains in a sample to how much would be in the same sample if it were made of entirely recently grown materials. The percentage is called the biobased content of the product. Samples are combusted in a quartz sample tube and the gaseous combustion products are transferred to a borosilicate break seal tube. In one method, liquid scintillation is used to count the relative amounts of carbon isotopes in the carbon dioxide in the gaseous combustion products. In a second method, 13C/12C and 14C/12C isotope ratios are counted (14C) and measured (13C/12C) using accelerator mass spectrometry. Zero percent 14C indicates the entire lack of 14C atoms in a material, thus indicating a fossil (for example, petroleum based) carbon source. One hundred percent 14C, after correction for the post-1950 bomb injection of 14C into the atmosphere, indicates a modern carbon source. ASTM D 6866 effectively distinguishes between biobased materials and petroleum derived materials in part because isotopic fractionation due to physiological processes, such as, for example, carbon dioxide transport within plants during photosynthesis, leads to specific isotopic ratios in natural or biobased compounds. By contrast, the 13C/12C carbon isotopic ratio of petroleum and petroleum derived products is different from the isotopic ratios in natural or bioderived compounds due to different chemical processes and isotopic fractionation during the generation of petroleum. In addition, radioactive decay of the unstable 14C carbon radioisotope leads to different isotope ratios in biobased products compared to petroleum products. As used herein, “biologically derived”, “bioderived”, and “biobased” may be used interchangeably to refer to materials whose carbon content is shown by ASTM D 6866, in whole or in significant part (for example, at least about 20 percent or more), to be derived from or based upon biological products or renewable agricultural materials (including but not limited to plant, animal and marine materials) or forestry materials.
Propylene glycol and ethylene glycol have, as mentioned previously, conventionally been produced from petrochemical sources. Commercial production of petroleum-based or -derived propylene glycol involves the hydration of propylene oxide, made predominantly by the oxidation of propylene. The commercial production of ethylene glycol similarly involves the hydration of ethylene oxide, made by the oxidation of ethylene. Propylene and ethylene in turn are industrial by-products of gasoline manufacture, for example, as by-products of fluid cracking of gas oils or steam cracking of hydrocarbons.
The world's supply of petroleum is, however, being depleted at an increasing rate. As the available supply of petroleum decreases or as the costs of acquiring and processing the petroleum increase, the manufacture of various chemical products derived therefrom (such as propylene glycol and ethylene glycol) will be made more difficult. Accordingly, in recent years much research has taken place to develop suitable biobased propylene glycol and ethylene glycol products, which can be interchangeable with propylene glycol and ethylene glycol products deriving from petroleum refining and processing methods but which are made from renewable versus nonrenewable materials.
As a result of these efforts, processes have been developed by several parties involving the hydrogenolysis of especially five and six carbon sugars and/or sugar alcohols, whereby the higher carbohydrates are broken into fragments of lower molecular weight to form compounds which belong to the glycol or polyol family. Sugars containing five carbon chains, such as ribose, arabinose, xylose and lyxose, lactic acid and five carbon chain sugar alcohols such as xylitol and arabinitol, are among the materials contemplated in U.S. Pat. No. 7,038,094 to Werpy et al., for example, while six carbon sugars such as glucose, galactose, maltose, lactose, sucrose, allose, altrose, mannose, gulose, idose and talose and six carbon chain sugar alcohols such as sorbitol are addressed by U.S. Pat. No. 6,841,085 to Werpy et al. (hereafter, “Werpy '085”) . Some of these carbohydrate-based feedstocks are commercially available as pure or purified materials. These materials may also be obtained as side-products or even waste products from other processes, such as corn processing. The sugar alcohols may also be intermediate products produced in the initial stage of hydrogenating a sugar.
For other known examples of such processes, U.S. Pat. No. 5,206,927 describes a homogeneous process for hydrocracking carbohydrates in the presence of a soluble transition metal catalyst to produce lower polyhydric alcohols. A carbohydrate is contacted with hydrogen in the presence of a soluble transition metal catalyst and a strong base at a temperature of from about 25° C. to about 200° C. and a pressure of from about 15 to about 3000 psi. However, as is evident from Tables II and III in the disclosure of U.S. Pat. No. 5,206,927, about 2-7% of other polyol compounds are produced in the hydrocracking process. U.S. Pat. No. 4,476,331 describes a two stage method of hydrocracking carbohydrates using a modified ruthenium catalyst. European Patent Applications EP-A-0523 014 and EP-A-0 415 202 describe a process for preparing lower polyhydric alcohols by catalytic hydrocracking of aqueous sucrose solutions at elevated temperature and pressure using a catalyst whose active material comprises the metals cobalt, copper and manganese. Still other examples of such carbohydrate-based processes may be found without difficulty by those skilled in the art.
Other efforts have been based on the use of another readily accessible biobased feedstock, namely, glycerol. Glycerol is currently produced as a byproduct in making biodiesel from vegetable and plant oils, through the transesterification reaction of lower alkanols with higher fatty acid triglycerides to yield lower alkyl esters of higher fatty acids and a substantial glycerol byproduct. Glycerol is also available as a by-product of the hydrolysis reaction of water with higher fatty acid triglycerides to yield soap and glycerol. The higher fatty acid triglycerides may derive from animal or vegetable (plant) sources, or from a combination of animal and vegetable sources as well known, and a variety of processes have been described or are known.
A biobased glycerol is also available as a product of the hydrogenolysis of sorbitol, as described in an exemplary process in U.S. Pat. No. 4,366,332, issued Dec. 28, 1982.
U.S. Pat. Nos. 5,276,181 and 5,214,219 describe a process of hydrogenolysis of glycerol using copper and zinc catalyst in addition to sulfided ruthenium catalyst at a pressure over 2100 psi and temperature between 240-270° C.
U.S. Pat. No. 5,616,817 describes a process of preparing 1,2-propanediol (more commonly, propylene glycol) by catalytic hydrogenolysis of glycerol at elevated temperature and pressure using a catalyst comprising the metals cobalt, copper, manganese and molybdenum.
German Patent DE 541362 describes the hydrogenolysis of glycerol with a nickel catalyst.
Persoa & Tundo (Ind. Eng. Chem. Res. 2005, 8535-8537) describe a process for converting glycerol to 1,2-propanediol by heating under low hydrogen pressure in presence of Raney nickel and a liquid phosphonium salt. Selectivities toward 1,2-propanediol as high as 93% were reported, but required using a pure glycerol and long reaction times (20 hrs).
Crabtree et al. (Hydrocarbon processing Feb 2006 pp 87-92) describe a phosphine/precious metal salt catalyst that permit a homogenous catalyst system for converting glycerol into 1,2-propanediol. However, low selectivity (20-30%) was reported.
Other reports indicate use of Raney copper (Montassier et al. Bull. Soc. Chim. Fr. 2 1989 148; Stud. Surf. Sci. Catal. 41 1988 165), copper on carbon (Montassier et al. J. Appl. Catal. A 121 1995 231)), copper-platinum and copper ruthenium (Montassier et al. J. Mol. Catal. 70 1991 65). U.S. Pat. No. 7,790,937 to Henkelmann et al. similarly describes converting a glycerol-containing stream, especially a glycerol-containing stream obtained from biodiesel production, to propylene glycol by reaction with hydrogen in the presence of a heterogeneous catalyst containing copper. Raney copper and copper-containing metal alloys in the form of a Raney catalyst are mentioned as preferred.
Still other homogenous catalyst systems such as tungsten and Group VIII metal-containing catalyst compositions have also been tried (U.S. Pat. No. 4,642,394). Miyazawa et al. (J. Catal. 240 2006 213-221) & Kusunoki et al (Catal. Comm. 6 2005 645-649) describe a Ru/C and ion exchange resin for conversion of glycerol in aqueous solution.
The previously-cited Werpy '085 reference contemplates conversion of a composition including glycerol to bioderived propylene glycol by reaction with hydrogen in the presence of a Re-containing multimetallic catalyst.
Numerous other examples of like processes may be found without difficulty by those skilled in the art.
One of the recognized problems in producing a biobased propylene glycol or ethylene glycol by any of these methods, however, is that other diol compounds are formed (e.g., four carbon and higher diols) to varying degrees in all of these processes. The boiling points of many of these materials are very close to one another, so that the separation of high purity bioderived propylene glycol from these other polyhydric alcohols is exceedingly difficult by conventional distillation methods—such that substantial amounts of the desired propylene glycol product are inevitably co-distilled with the higher diols in order to remove these to the extent needed for many commercial applications.
Several reports in the literature describe efforts for azeotropically separating the other polyhydric alcohols from propylene glycol. For instance, U.S. Pat. No. 4,935,102 describes a method for using an azeotrope forming agent such as propylene glycol isobutyl ether, tetrahydrofurfuryl alcohol, N,N-dimethylacetamide, ethylene glycol diethyl ether, diethylene glycol diethyl ether, 2-methoxyethyl ether, ethylene glycol n-butyl ether, diacetone alcohol and ethyl n-butyl ketone. In U.S. Pat. No. 5,423,955, the azeotrope forming agent consists of a material selected from the group consisting of toluene, ethyl benzene, o-xylene, p-xylene, cumene, m-diisopropyl benzene, m-diethyl benzene, mesitylene, p-cymene, hexane, cyclohexane, methyl cyclohexane, heptane, 3-methyl pentane, octane, decane, 2,3,4-trimethyl pentane, dipentene, decalin, dicyclopentadiene, alpha-phellandrene, limonene, hemimellitene, myrcene, terpinolene, p-mentha-1,5-diene, beta-pinene, 3-carene, 1-heptene, cyclopentane, pentane, o-diethyl benzene, 2,2-dimethyl butane and 2-methyl butane.
Alternative approaches to purifying the product mixture have been proposed in commonly-assigned United States Patent Application Publication US 2008/0275277A1 to Kalagias, published Nov. 6, 2008, wherein the addition of a polar solvent and extractive distillation are presented as an alternative to the use of an azeotropic agent, and in commonly-assigned United States Patent Application Publication US2009/0120878A1 to Hilaly et al., published May 14, 2009, wherein simulated moving bed chromatography is offered as a means to achieve a purified, commercial grade biobased propylene glycol.
Nevertheless, the separation of a number of byproducts from the desired bioderived propylene glycol remains difficult and costly. The presence of four carbon and higher diols can in particular mean substantial losses of co-distilled propylene glycol product where conventional distillation methods are desired to be used for product purification, so that a process which enables a lesser amount of byproduct higher diols would be welcomed.